Heating
Heat-Only Boiler
In short
Heat-Only Boiler: Boiler with 4-core connection (L/N/E/SL). It has 5 terminals: L (Live), N (Neutral), E (Earth), SL (Switched Live), PL (Pump Live).
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
Terminals
| Terminal | What it is | On an S-Plan | On a Y-Plan |
|---|---|---|---|
| L | Live | Wiring centre terminal 1 (permanent live) | Wiring centre terminal 1 (permanent live) |
| N | Neutral | Wiring centre terminal 2 (neutral) | Wiring centre terminal 2 (neutral) |
| E | Earth | Wiring centre terminal 3 (earth) | Wiring centre terminal 3 (earth) |
| SL | Switched Live | Wiring centre terminal 10 (switched live from both valve oranges) | Wiring centre terminal 8 (switched live from the cylinder stat call and the valve orange) |
| PL | Pump Live | Pump L (pump live) | Pump L (pump live) |
How it fails
- Circulating pump seized
The pump has 230 V across L-N but does not run: seized. The boiler overheats and locks out.
- Boiler switched live open circuit
Both zones make the switched live at the wiring centre, but it never reaches the boiler's SL: broken conductor.
- Intermittent break in the boiler switched live
The demand is steady at the wiring centre but flickers at the boiler's SL: a conductor breaking down intermittently.
- Boiler switched live landed on permanent live
The boiler's SL is landed on the permanent-live terminal, so it fires with no demand at all - no boiler interlock.
- Boiler live and neutral reversed
L-N 230 V, L-E 0 V, N-E 230 V: reversed polarity at the boiler. It will not run until L and N are the right way round.
Where it connects
Lessons that use it
- Welcome to Draw Warm
Getting Started
- Using the Tools
Getting Started
- Connecting Wires
Getting Started
- Using the Multimeter
Getting Started
- Introduction to Mains Electricity
Electrical Fundamentals
- Permanent Live vs Switched Live
Electrical Fundamentals
- The Boiler
Component Library
- Boiler Connections
S-Plan Wiring
Systems it is part of
Also in heating
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